I dont' know how else to 'splain it. It's something that has started recently. Seems like when I'm going slowly (thankfully) but when I start to push down on the pedal, it almost seems like the pedal is pushing back with a feel (and sound) like the ABS is kicking it back at me and it goes down VERY hard. Other times the brakes work as normal.

The first time I felt it, I was on dirt with the wheels cranked and figured maybe the ABS was kicking in because a tire was slipping. Well, it did it on pavement yesterday, I was pulling into a driveway. I'd slowed fine to make the turn and then as I was coming to a stop, I felt it again.

I'm wondering if an ABS sensor is acting up? Would something like that throw a code at me?

 

Almost sounds like not enough pressure is coming out of your power steering pump not helping with the hydro boost.
Is steering hard at the same time the brakes are hard?
Are you turning sharp when the brakes turn hard?

 

Bill,
Only once was I in a "turning" situation. Otherwise the engine was near idle and front wheels were straight ahead. I really haven't noticed that the steering wheel was hard to move but hten I haven't been looking for it.

 

Wow... No one? Nothing?

The only commonality I can find is that it happens at very low speeds.

 

Well low speed=pump not turning fast=not much pressure helping power brakes. You also have an abs relay in your PDC under the hood. Remove it to disable abs and see if it goes away. Check underneath the truck at the underside of the power steering pump/vacuum pump to see if it's wet. Could be the seal gone. I recently started my truck to find I had no steering or brakes and a puddle of fluid on the ground. I did the fixinram's vacuum pump repair. Worked great. Hope this helps.

 

also check the sensor wires coming from wheel bearings for a bad spot but my bet is on the power steerig pump pressure because my 01 is doing something just like that and my steering is getting harder

 

http://www.turbodieselregister.com/f...ssist-fix.html

Try this, it saved my friend the price of a new P.S. pump. Hope it helps you out.

Floyd

 

Clean the screen

 

I dont seem to have any issues with steering, even on dry pavement, not moving. Will crawl under to see if I have any wet tho. I was wondering if it was a bad abs sensor at a wheel? Is the ABS relay clearly marked?

 

That's funny, I had the exact same problem for the first time last night and again once this morning. It was the reason I signed up here...

Anyhow, I'm 100% convinced it is not lack of pressure from the hydroboost system. I've had vacuum boosters leak / fail and it's nothing like this. To say the pedal is 'hard' to push is almost incorrect. In my case it was darn near impossible to push. Felt like I was trying to shove a 2x4 through the floorboards...

I am leaning towards the ABS system going a little haywire. If for some reason the ABS system thought that all the wheels were in lockup it could do this. The pump would be busy returning fluid to the master cylinder, that in normal operation is being dumped from the calipers. In this case the pump is returning fluid to an already full master cylinder, there by increasing the pressure inside the master cylinder and not allowing the brake pedal to move. The brakes would not be getting any fluid because it's all being returned through the ABS system. The other reason I am leaning towards ABS is that lately I've had a few occurences when it turned on when it wasn't necessary (i.e. low speed stops on dry pavement). It hasn't happened in a while but the ABS light used to occasionally flash when it was raining out.

The only other thought I had was that some debris found it's way into the master cylinder and was, at a certain position, blocking the ports so no fluid could get to the brakes. This doesn't explain why the ABS was cycling and would affect both front and rear hydraulic circuits.

As I haven't found any other info my game plan at the moment is to thouroughly inspect all the wheel speed sensor wiring, clean contacts and such. If it acts up after that I may pull the ABS fuses and see if it goes away. At least get it narrowed down.

 

If the pedal was hard to push all the time I would suspect the hydroboost. A hard pedal is the classic symptom since there is no "Power Boost" added to your foot pedal. In your case it is not.

 

I am thinking ABS as well. I haven't looked for the relay yet but I am going to although I do not recall my ABS light flashing at any point. Is there a code that would be thrown here?

 

It would not be an ABS issue, the difference is if you have an ABS issue the truck would not stop or have one wheel lock and one unlock, but still have power assisted brakes. good pedal feel but a poorly stopping truck. worse case is if you have ABS failure your truck reverts to normal brakes, not hard brakes

The hard pedal is the same as having NO assist. it is a pump issue or a faulty hydroboost.

1) to test the HB start truck, shut it off then pump the brakes, you should get 2-3 brake applications before the pedal gets hard.(any less its a bad HB)

2) Start the truck (with foot still on the brake pedal after doing step 1) power assist should return within a couple of seconds , if not its likely a very weak power steering pump

3) start the truck, shut the truck down, do not touch the brake pedal for 3-4hrs or till the next day. hop in the truck and hit the brakes with the engine off, again you should have 2-3 brake applications before the brakes get hard, if the brakes are hard when you first step on them the HB is faulty.

4) last test, is to turn the wheel back and forth (truck not moving) so that the Power steering pump is loaded up, pump the brakes repeatedly, if the steering or the brakes get hard the pump is weak and needs replacing.

OK you can do tests 3 & 4 in reverse order.

sometimes you can get lucky and flush the Power steering fluid or replace it with synthetic fluid and it will cure the problem. as PS fluid can and does degrade over time

I would not waste my time chasing ABS faults, until you have diagnoised the hydroboost stuff first.


68 Bronco,
I cut this from your post
***If for some reason the ABS system thought that all the wheels were in lockup it could do this. The pump would be busy returning fluid to the master cylinder, that in normal operation is being dumped from the calipers. In this case the pump is returning fluid to an already full master cylinder, there by increasing the pressure inside the master cylinder and not allowing the brake pedal to move. The brakes would not be getting any fluid because it's all being returned through the ABS system. *****

The fluid is not returned to the master cyl during ABS stops. what actually happens is the ABS interupts fluid from going to each wheel, when the ECM sees that the wheel is turning again the ABS pump builds pressure and "locks" the wheel again, this cycle is repeated very quickly to keep the wheel from "locking up"

There cannot be any pressure in the Master cyl resivor because if there was the brakes would be LOCKED on. there is a small hole(port) in the resivor that feeds the master cyl bore and as the brake is applied a piston blocks off the feed hole sealing the fluid from the resivor and pressurizing the fluid trapped between the moving piston (hooked to break pedal) and all the way to the calipers and wheel cyls, when you release the brakes the piston moves back allowing the fluid to return to the master cyl via the feed hole.
So... as long as your foot is on the pedal (brakes applied) it would not matter if the Master cyl was over filled or pressurised because the fill port is blocked by the piston and nothing can get in. it would ONLY matter if your brakes are not applied, as the port would be open and the "pressure" would apply the brakes, I think a diagram/picture would explain this much better than I'm trying to describe

as for a flashing abs light, best to pull the codes to help narrow that down, likely the symptoms you describe are not related.

 

Thanks for the suggested tests for the booster. The truck passes all of them. A booster issue would not explain, at least in my view, why the ABS was active and that it is an intermitant problem.

Perhaps you misunderstood me, I did not mean the master cylinder resevoir but the master cylinder itself. On our trucks the ABS pump does not pressurize the brakes during an ABS event. It's sole task is to return the fluid dumped from the brakes to an accumulator and then back to the pressure side of the master cylinder, not the fluid resevoir, you WILL feel this happening and the pedal will get hard. I happen to work with the guy who did the ABS tuning on the '01-'02 rear disk trucks and I confirmed this with him. On other systems, mainly those with traction control, the motor can apply the brakes independantly of the brake pedal, but not on our trucks

His line of thinking is it may actually be the DRP (dynamic rear proportioning) being over sensitive. I will check into the rear brakes some and see if perhaps the parking brake or a caliper is hanging up.

For those who are curios, DRP operates like a highly sensitive ABS system. It looks for very minor differences in front & rear wheel speed (much lower than will trigger ABS). Once it sees this it will modulate pressure to the rear brakes to avoid a real wheel lock condition. This allows them to run very large rear brakes without worry of premature rear lock-up when the truck is unloaded.

 

Fixed!! The right front wheel speed sensor was out of whack. Did not trip the ABS light and had no visual clues from the outside that anything was amiss with it (cracking , insulation worn through etc.)

After some further thought I ruled out trouble with the DRP system as you would not hear the ABS pump run, which at that time I had not. Subsequent occurences I listened for it and sure enough it was running.

XP8103 - Assuming it's the same problem I had you'll want to look into the wheel speed sensors.